This invention relates generally to a process for the treatment of oil-containing or oil-producing solids to extract fuel gases and liquid crude oil products therefrom. More particularly, the invention relates to a process for retorting arsenic-containing oil shale so as to produce a liquid shale oil which has a significantly lower arsenic content.
Vast deposits of oil shale, a sedimentary inorganic rock containing about 35 weight-percent calcite (CaCO.sub.3), 15 weight-percent dolomite (MgCO.sub.3.CaCO.sub.3), and 10 weight-percent alkali metal salts are known to exist in the United States, especially in the Green River formation in Colorado, Utah, and Wyoming. The oil shale in these deposits contains between 5 and 35 weight-percent of hydrocarbons in a form known as kerogen. When pyrolized, this kerogen decomposes to produce crude shale oil vapors, which, upon condensation, become a valuable source of fuel.
Several pyrolytic processes have heretofore been developed to produce crude shale oil from oil shale. One such process is shown in U.S. Pat. No. 3,361,644, which is incorporated herein by reference. In this process oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving oil shale continuously exchanges heat with a downwardly flowing, high-specific-heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 1200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the oil shale. In the lower section (the preheating zone), the oil shale is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed hydrocarbonaceous vapors plus hydrogen. Most of the heavier hydrocarbons condense in this lower section and are collected at the bottom of the retort as a product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain more product oil. The remaining gases are then utilized as a product gas, a recycle gas as hereinbefore described, and a fuel gas to heat the recycle gas to the hereinbefore specified temperature of 1200.degree. F.
A problem with this and all similar oil shale retorting processes is that, during retorting, arsenic components present in oil shale either sublime to or are pyrolyzed into vaporous arsenic-containing components. As a result, arsenic in various forms collects with the educed hydrocarbonaceous vapors and condense with the heavier hydrocarbons in the preheating zone, or, in some processes, in a condenser situated outside of the retorting vessel. When oil shale obtained from the Green River formation is retorted, the concentration of arsenic in the produced crude shale oil is usually in the range of 30-80 ppmw. But since crudes containing such high concentrations of arsenic present problems in refining, especially with respect to poisoning hydrocarbon conversion catalysts used in catalytic cracking, hydrotreating, hydrocracking, reforming, etc., and since such oils also present an obvious pollution problem if burned without refining, the necessity for removing the arsenic from crude shale oil, or preventing its formation as vaporous components in the retorting zone, is clear.
However, presently available methods devised to produce an arsenic-free shale oil involve removing the arsenic from the liquid shale oil obtained from the retort. One such method is shown in my U.S. application Ser. No. 700,017 filed June 25, 1976, now U.S. Pat. No. 4,046,674 wherein arsenic-containing shale oil is contacted with an absorbent containing nickel sulfide, molybdenum sulfide, and alumina, under conditions of elevated pressure and temperature so as to obtain an arsenic-free shale oil. But although such a process is effective for removing arsenic from shale oil, it obviously would be more desirable to prevent the formation, or to minimize the amount, of vaporous arsenic components produced in the retorting zone. But no process for producing such a result is commercially available.